An Operating System is the low-level software that manages a computer. It controls the way the hardware operates, the communication with peripheral devices such as a monitor, a printer or a keyboard, the allocation of memory between various applications, along with the prioritization of apps when several run simultaneously. Any program which is installed on a desktop or a notebook works by sending requests to the OS for different services using an application program interface (API). The contact with the OS can be performed using a Graphical User Interface (GUI) or a command line. In the hosting world, the Operating System is what commands a hosting server along with all the software installed on it, which includes not only website scripts, but, in addition any other application for instance a VOIP or a game server. If there are virtual machines created, they employ a guest OS which is run on the server host OS.